1. Field of the Invention
This invention relates to a translucent, co-extruded thermoplastic sheet having use as a light diffusing protective cover or sign face for light emitting diode (LED) light sources and for other purposes and to LED signs using such sheets.
2. Prior Art
The most common plastics used as the face of illuminated signs and channel letters are acrylic and polycarbonate. Acrylics are typically used when high luminous transmission, excellent weatherability and/or resistance to yellowing is required. Polycarbonate is commonly used when high impact, heat or flammability resistance properties are required. In many of these applications the plastic is pigmented to achieve the desired color and transmission properties.
The most common light source used in illuminated channel letter applications has historically been luminous tubes. Luminous tubes are glass tubes filled with a gas which when subjected to high voltage (typically 3,000-15,000 Volts), becomes luminescent in a color that is characteristic of the gas used, of the gas and the color of the glass tubing combined, or of the fluorescent phosphors coating the inner wall of the glass tube. Neon gas emits a red color and argon gas mixed with mercury vapor emits a blue color, for example. These luminous tubes are typically 9 to 15 millimeters in diameter and are contoured to conform to the desired letter shapes.
A more recent trend in illuminating channel letters is to explore the possibility of switching from luminous tubes filled with a gas, particularly neon, to light emitting diode (LED) light sources. The reasons for sign makers to explore the use of LED light sources are as follows. LEDs have a longer expected lifetime (up to 100,000 hours), operate at low voltages (12 Volts DC), and reportedly offer greatly reduced operating costs. The reduced operating costs offered by LED light sources are attributable to the low voltage operating requirements, the highly directional luminous output and the very narrow wavelength range(i.e., a range of about 50 nanometers is possible) of the luminous output. These reduced operating costs not only benefit the end-users of the signs but the wide-spread use of LEDs may also help to significantly reduce energy consumption.
A comparison of the two light sources (luminous tubes and LEDs) reveals that they are very different. The luminous tubes may be very long and continuous in nature as contrasted with LEDs which are discontinuous and closer to representing point sources of light. Another difference is that the output from a luminous tube is typically emitted at all angles (0-360 degrees) radially from the tube whereas the LED output is highly directional and more wavelength specific.
When channel letter manufacturers have attempted to replace the luminous tubes with LED light sources, they have discovered that some of the plastic faces (particularly the lighter colors) do not hide the LED sources. They have also observed that the light is not distributed evenly across the face of the channel letter when using the LED light sources. In order to correct this problem, secondary operations such as applying a film to the light source side of the plastic face has been tried. This type of secondary operation may improve the ability of the face to hide the LED light sources but it also visibly changes the luminous transmission and transmitted color properties of the resultant plastic face when illuminated, and the reflected color when it is not illuminated. Additionally, this type of secondary operation is costly and creates yet another problem for sign makers, as described below.
Frequently there are multiple signs at one company location and many times there are multiple sign types. For instance, it is common to see a channel letter sign on a building advertising the name of the company and also a large stand-alone, street sign on a pole on the same property. It is often specified that the street sign face, in the area that advertises the company name, and the channel letter sign face, have the same optical properties (luminous transmission, transmitted and reflected color, and gloss). Hence, quite frequently the sign face material for each sign is the same. Therefore, when neither sign is illuminated, the reflected color and gloss properties of the sign faces are the same. Furthermore, when the signs are illuminated, the luminous transmission (i.e., brightness), and the transmitted colors of the sign faces are the same or similar. Note that the transmitted colors above may not be exactly the same when illuminated if there are differences in the color emission characteristics of the light sources (i.e., luminous tube versus fluorescent tube). Now if the sign maker changes from a luminous tube, light source to LEDs and applies a film on one side of the plastic sign face as previously described, the transmitted and reflected colors of the channel letter sign no longer match those of the street sign face. This poses yet another problem to the sign maker.
It is known that a secondary operation such as sand blasting one surface of the plastic sheet will cause sufficient surface roughness to hide an illuminated LED. However, this is a costly secondary processing step that leads to inherently high levels of product inconsistencies.
Standard acrylic sign face covers are commonly used in sign applications with luminous tube light sources. Examples of these include Plexiglas® MC (an extruded and melt calendered acrylic sheet comprised of a MMA/EA copolymer having a melt flow rate of about 2 g/10 minutes when measured in conformance with ASTM D 1238, Condition 230° C./3.8 kg, procedure A) sheet colors. It is also known that plastic sheet manufacturers may emboss a matte finish pattern onto the surface of the sheet during the extrusion process. The state-of-the-art method for producing plastic matte finish surfaces via a sheet extrusion process is to use an embossing roll to impart a matte finish pattern into the molten polymer extrudate as it is being polished and cooled into sheet. This is another technique that may be used to achieve the desired light scattering properties.
However, this embossing technology in the sheet extrusion process has several shortcomings:                1. The extrusion line needs to be shut down in order to change one of the polishing rolls from a typically used highly polished chrome roll used to produce standard products such as Plexiglas® MC sheet to a patterned roll typically used for a specialty product such as Plexiglas® MC Matte Finish sheet.        2. The degree of matte finish or gloss of the resultant Plexiglas® MC Matte Finish sheet changes dramatically for each sheet thickness produced. Typically with this embossing technology, a much higher gloss results on the thicker gauge sheet. This is because the thicker sheet retains the heat necessary to extrude or melt calendar the material for a much longer period of time. Consequently, after the sheet has been embossed with the pattern, the thicker gauge sheet tends to relax more and deviate more from the original embossed pattern. Hence, the amount of light scattering to hide an LED is not consistent with changes in sheet thicknesses.        
The present invention enables an acrylic sheet to be economically produced using a co-extrusion process wherein:                1. The line does not need to be shut down to change from a highly polished sheet product to a specialty matte finish sheet product.        2. The gloss properties of the resultant sheet at various thicknesses are more consistent when compared with sheet produced using the embossing technology.        
The present invention also enables a plastic sheet to be economically produced using the co-extrusion process wherein:                it effectively hides LED light sources and distributes the light,        the sign maker may change the light source from a luminous tube to LEDs in channel letters without significantly changing the following properties:        the reflected color of the sign face exterior        the gloss of the sign face exterior        the transmitted color of the illuminated sign face        the luminous transmission of the illuminated sign face.        
This invention broadens the range of channel letter/sign face colors that may be used in applications illuminated with LEDs or other types of point light sources. It is only by the strategic positioning of these particles on the interior surface or as an interlayer of a sign face that these optical properties may be achieved.